DST migration analyses

When do tag 308 and 321 start/stop migrating?
Published

March 20, 2023

Research Question: can migration and residency behaviour be inferred from the depth log?

vertical space use

Depth median, min and max

  • There are peaks in the change of daily median in September, December (this is influenced by the non-accurate depth measurements, though), March and May.

  • There seems to be no clearly visible pattern in daily vertical migration, comparing between potentially migrating and residential periods.

  • These months I will have a further look into.

  • Here we see peaks in the change of daily median in October 2018, May 2019, August 2019 and October 2019.

  • Both in October 2018 and 2019, we see reverse Diel Vertical Migration.

Note: a rolling mean over 3 days was applied to smooth the data.

Daily Vertical Migration

  • Diel Vertical Migration (DVM): Depth during day significantly deeper than during night.
  • reverse DVM (rDVM): Depth during night significantly deeper than during day.
  • no DVM (nDVM): No significant difference between depth during night and day.

There does not seem to be a pattern between potential activity (migrating/residing) and Daily Vertical Migration behaviour.

Vertical speed vs depth

Vertical speed = the absolute difference between two subsequent measurements divided by the number of samples taken per minute (Units = m/min).

  • There is a bimodal distribution in monthly vertical speed, no matter the depth that was swum in and the potential activity (migrating/resting).

Potential Migration months

Sep-Oct

The interesting period is between Sep 20 and October 15.

Oct-Nov

Horizontal distance

We need to be careful interpreting this since it’s a model output.

  • Overall, the female shark (tag 308) travelled 1462.54 km and the male shark (tag 321) travelled 1272.07 km.

  • The horizontal distance covered by the female shark (tag 308) is 536.52 km, and for the male shark (tag 321) it is 173.93.

You can select a zoom window, and select/deselect data by clicking on then on the legend.

  • the largest horizontal distance travelled per day is roughly 30 km.

k means clustering

Can unsupervised learning distinguish migrating and resident behaviour?

  • PCA on daily dst summary statistics (median depth, depth range, vertical speed, …), retain the forst 2 PC axes.

  • the k means clustering I did can distinguish between summer and winter residential behaviour, but is not able to differentiate the migrating periods in between.

Map